Passive Ionospheric Sounding and Ranging

Background

In previous experiments, Peter G3PLX has used Digital Signal Processing
(DSP) and doppler techniques to measure small differences in carrier
frequency that result from movement in the radio propagation path. While
interesting for meteor scatter, aircraft and satellite reflections, and more
gross ionospheric effects, in using this technique it is not possible to
infer information about a relatively static propagation medium. Peter
realised that what was required was a time domain
- rather than frequency domain - technique, for example measuring the
propagation of pulses. It was soon established that a wideband technique,
rather than a carrier based technique would be necessary.

In searching for suitable pulse transmissions to use, preferably
transmissions available from all over the world on a 24 hour basis, Peter
stumbled across a family a pulse generators that are used as swept frequency
ionospheric sounders. In their normal application, research, professional
and military groups use these low power devices to probe the ionosphere to
measure propagation.

The signal consists of a single long "chirp", sweeping
up in frequency at a constant rate. These transmissions are tracked by a
companion receiver which is zero beat with the transmitter, and so
ionospheric reflections that are returned with short delays are heard as
lower sideband audio beats of a few hundred Hz. The equipment then builds an
"ionogram" or two dimensional graphical representation of the ionosphere's
reflection height or delay against frequency. The adjacent picture
illustrates a typical commercial 50W FM/CW (chirped) ionospheric sounding
transmitter.

The first step was to discover how these chirped signals could be used in a
passive manner, i.e. without reference to the transmitter oscillators or
timing reference. To do this, Peter developed a very clever chirped filter,
which not only sweeps in frequency at 100 kHz/second, but has properties not
possible in a conventional filter - a bandwidth of only 66 Hz, but a pulse
resolution of 0.66ms. This filter and matching detection software formed the
basis of the adventure to follow. 21476